First experience with third-party validations: A robust calibration and QA procedure for proton FLASH delivery

Proton FLASH radiotherapy, delivering ultra-high dose rates, shows promise in reducing normal tissue toxicity while maintaining tumor control. However, accurate dosimetry and quality assurance (QA) for FLASH remain challenging due to the extreme dose rates involved. Developing reliable calibration and QA procedures is crucial for advancing FLASH towards clinical implementation.

To present an effective routine calibration and QA procedure for proton FLASH delivery to ensure high-quality dosimetry performance for preclinical and clinical delivery.

A high temporospatial resolution strip ionization chamber array (SICA) detector was mounted to the treatment nozzle, which was calibrated using an Advanced Markus ion chamber (IC) under variable dose and ultra-high dose rates. The calibration curve was used to monitor the delivery in real-time. The maximum beam current was 215 nA at the isocenter at the highest energy of the cyclotron (250 MeV). Third-party IROC (Imaging and Radiation Oncology Core) thermoluminescent dosimeters (TLDs) were requested to verify the accuracy and effectiveness of the calibration and validation procedure. A transmission field with a field size of 3 × 3 cm² was designed to deliver 2000 cGy doses with a field-averaged dose rate varying from 36 to 76.5 Gy/s.

The entire calibration process took less than 10 minutes as part of routine daily QA. The calibration curve between the SICA detector and IC demonstrated an R-value of almost 1.00. The SICA detector measured each delivery, providing critical data for FLASH analysis, including field size, dose, delivery time, and dose rate. Based on real-time dose and dose rate monitoring, the planned and delivered doses were within 1% accuracy according to third-party TLD measurements.

As demonstrated by IROC measurements, effective calibration and QA using a SICA detector for proton FLASH delivery can ensure high accuracy in real-time dose and dose rate monitoring, paving the way for future clinical applications.